1. Field of the Invention
The present invention relates to a semiconductor device, which includes a driver circuit for driving a capacitive load based on an input signal, and an power supply transistor for supplying a power supply voltage to said driver circuit. In particular, it relates to a semiconductor device such as a driver circuit for a common terminal of a liquid crystal display device, where the output signal amplitude of said driver circuit needs to be adjusted based on the power source voltage.
2. Description of the Prior Art
An example of a conventional semiconductor integrated circuit device of this type is described below while referencing FIG. 8. FIG. 8 is a semiconductor integrated circuit device for driving a common terminal of a liquid crystal display device used in, for example, mobile telephones. In FIG. 8, 100 denotes to a semiconductor integrated circuit device, wherein external power source VCC is connected to power source terminal 1, ground terminal 2 is connected to ground, input signal Vin is supplied to input terminal 3, and capacitive load CL is connected to output terminal 4. Semiconductor integrated circuit device 100 includes internal power source circuit 10, which generates a desired internal power source voltage VHL from external power source voltage VCC, and switching circuit 20, which performs switching control in conformity with input signal Vin outputting internal power source voltage VHL to output terminal 4.
Internal power source circuit 10 is configured such that external power source voltage VCC is divided into a desired voltage level at the connection point of resistor 11 and resistor 12, which are connected in series, and is output as internal power source voltage VHL to switching circuit 20 via operational amplifier 13. As shown by the specific structural example illustrated in FIG. 9, operational amplifier 13 is configured such that it has P-channel MOSFET 14 and N-channel MOSFET 15, which are connected in series, at the output stage; the source of MOSFET 14 is connected to power source terminal 1; the source of MOSFET 15 is connected to ground terminal 2; and internal power source voltage VHL is output from the junction of MOSFET 14 and MOSFET 15.
Switching circuit 20 is configured such that it has a CMOS structure comprising P-channel MOSFET 21 and N-channel MOSFET 22, wherein internal power source voltage VHL is supplied to the source of MOSFET 21 and ground terminal 2 is connected to the source of MOSFET 22. With this structure, when input signal Vin, which is from input terminal 3, is supplied to the respective gates of MOSFET 21 and MOSFET 22 via inverter 23, MOSFET 21 and MOSFET 22 are on/off controlled to output internal power source voltage VHL from the junction of MOSFET 21 and MOSFET 22 to output terminal 4 as output voltage Vout.
As for the operation of semiconductor integrated circuit device 100 having the above-mentioned structure, once external power source voltage VCC is supplied to power source terminal 1, external power source voltage VCC is divided into a desired voltage level at the junction of resistor 11 and resistor 12, and the resulting voltage is output from internal power supply circuit 10 as internal power supply voltage VHL via operational amplifier 13. While in the state where internal power supply voltage VHL is being output from internal power supply circuit 10, as input signal Vin goes from a xe2x80x9cL=0xe2x80x9d level to a xe2x80x9cH=VCCxe2x80x9d level and MOSFET 21 of switching circuit 20 turns on, as shown in FIG. 10, electrical current flows from internal power source 10 via MOSFET 21 to capacitive load CL, which is connected to output terminal 4, and output voltage Vout climbs up to internal power source voltage VHL.
In the semiconductor integrated circuit device described above, while output voltage Vout climbs up to input power source voltage VHL, the electric potential at the gate of MOSFET 21 is at level xe2x80x9cLxe2x80x9d, and MOSFET 21 is turned on completely. On the other hand, while operational amplifier 13 of internal power source circuit 10 is in the state where external.power source voltage VCC is divided at the junction of resistor 11 and resistor 12 thereby being supplied to the non-inverting input terminal, since the electric potential at the gate of MOSFET 14 does not reach level xe2x80x9cL=0xe2x80x9d, even though MOSFET 14 may be on, it is not turned on completely. At this point, for example, if the size of MOSFET 14 is designed to be the same size as MOSFET 21, although MOSFET 21 may be turned on completely, since MOSFET 14 is not turned on completely, the electric current capacity of MOSFET 14 is insufficient when compared to the electric current capacity of MOSFET 21. In this case, the operating speed of operational amplifier 13 in response to changes in electric current is slow and cannot keep up with high-speed electric current changes occurring during switching. Accordingly, as shown in FIG. 10, there is a problem, wherein after internal power source voltage VHL has temporarily fallen, since the rising waveform up to the desired voltage has a gentle slope, the rising waveform of the output voltage Vout has also a gentle slope.
When a semiconductor integrated circuit device is used as a driver of liquid crystal display device, the desired display cannot be achieved. In order to speed up the operational speed in response to electric current changes in operational amplifier 13, a larger size of output transistor 14 included in operational amplifier 13 may be used; however, there is a problem with the semiconductor integrated circuit device having a larger chip size.
The present invention has come about in consideration of the problems described above and aims to provide a semiconductor integrated circuit device in which switching speed is increased by increasing electric current capacity of an internal power source circuit the instant a switching circuit is switched on.
A semiconductor integrated circuit device according to the present invention, comprises:
a driver circuit driving a capacitive load responding to an input signal;
a power source transistor biased so as to supply a fixed voltage to a power supply terminal of said drive circuit; and
a control circuit detecting a change in said input signal, forcibly shifting bias of said power source transistor and increasing an electrical current supply capacity of said power source transistor when said input signal changes to drive said capacitive load with electric current supplied from said power source transistor.